diff --git a/web/app/[locale]/layout.tsx b/web/app/[locale]/layout.tsx
index 3a69af7b1a..6e14eb877b 100644
--- a/web/app/[locale]/layout.tsx
+++ b/web/app/[locale]/layout.tsx
@@ -1,10 +1,11 @@
 import type { Metadata } from "next";
 import localFont from "next/font/local";
+import { notFound } from "next/navigation";
 import { Nav } from "@/components/nav";
 import { Footer } from "@/components/footer";
 import { UsageCounting } from "@/components/usage-counting";
 import { BUILD_FACTS } from "@/lib/facts";
-import { localeDirection, locales, type Locale } from "@/lib/i18n/config";
+import { isValidLocale, localeDirection, locales, type Locale } from "@/lib/i18n/config";
 import { getChrome, getHome } from "@/lib/i18n/dictionaries";
 import { serializeJsonLd } from "@/lib/json-ld";
 import { buildPageMetadata } from "@/lib/page-meta";
@@ -74,6 +75,13 @@ export default async function LocaleLayout({
   params: Promise<{ locale: string }>;
 }) {
   const { locale } = await params;
+  // A single-segment URL that is not a routed locale — e.g. a stray dotted
+  // path such as /foo.txt, which middleware deliberately leaves alone so real
+  // static files keep resolving — would otherwise bind `[locale]` to that
+  // segment and render the shared home page as a 200 under a fake locale
+  // (`lang="foo.txt"`). An unregistered locale is not a page: answer with an
+  // honest 404 and let the not-found boundary render instead.
+  if (!isValidLocale(locale)) notFound();
   const chrome = getChrome(locale);
   // RTL locales (e.g. ar) set the document direction from the canonical
   // registry so the browser handles bidirectional layout from the root.
diff --git a/web/lib/i18n/locale-layout-guard.test.ts b/web/lib/i18n/locale-layout-guard.test.ts
new file mode 100644
index 0000000000..4593726c30
--- /dev/null
+++ b/web/lib/i18n/locale-layout-guard.test.ts
@@ -0,0 +1,27 @@
+import { readFileSync } from "node:fs";
+import { describe, expect, it } from "vitest";
+
+/**
+ * A single-segment URL that is not a routed locale — a stray dotted path such
+ * as /foo.txt, which middleware deliberately leaves alone so real static files
+ * keep resolving — binds `[locale]` to that segment and would otherwise render
+ * the shared home page as a 200 under a fake locale (`lang="foo.txt"`): a
+ * soft 404 for crawlers and generative-engine probes. The layout must turn any
+ * locale outside the routed registry into a real 404 before it renders.
+ */
+describe("locale layout rejects unregistered locales", () => {
+  const layout = readFileSync(new URL("../../app/[locale]/layout.tsx", import.meta.url), "utf8");
+
+  it("guards on isValidLocale and calls notFound()", () => {
+    expect(layout).toContain("import { isValidLocale, localeDirection, locales, type Locale }");
+    expect(layout).toContain("if (!isValidLocale(locale)) notFound();");
+  });
+
+  it("checks the locale before reading dictionaries or rendering chrome", () => {
+    const guard = layout.indexOf("isValidLocale(locale)");
+    const chrome = layout.indexOf("getChrome(locale)");
+    expect(guard).toBeGreaterThan(-1);
+    expect(chrome).toBeGreaterThan(-1);
+    expect(guard).toBeLessThan(chrome);
+  });
+});